Abstract
Facial expression is a mode of closeโproximity nonโvocal communication used by primates and is produced by mimetic/facial musculature. Arguably, primates make the mostโintricate facial displays and have some of the mostโcomplex facial musculature of all mammals. Most of the earlier ideas of primate mimetic musculature, involving its function in facial displays and its evolution, were essentially linear โscala naturalโ models of increasing complexity. Moreโrecent work has challenged these ideas, suggesting that ecological factors and social systems have played a much larger role in explaining the diversity of structures than previously believed. The present review synthesizes the evidence from gross muscular, microanatomical, behavioral and neurobiological studies in order to provide a preliminary analysis of the factors responsible for the evolution of primate facial musculature with comparisons to general mammals. In addition, the unique structure, function and evolution of human mimetic musculature are discussed, along with the potential influential roles of human speech and eye gaze. BioEssays 30:212โ225, 2008. ยฉ 2008 Wiley Periodicals, Inc.